The use of nitroso derivatives in rubber compounds leads to a reinforcing effect with important practical implications. The mechanism of action of these promoters is explained on the basis of the theory according to which the reinforcing effect is due to the strain-crystallization process. This process is dependent on various factors, one of them being the presence of a network based on weak “rubber…carbon black” bonds. The properties of crude and cured blends on natural rubber, polyisoprene, and modified polyisoprene are compared. When nitroso derivatives are used, the active sites on the polymer chain are probably the nitroxyl radicals formed during the modification process. According to the mechanism proposed by the authors in previous papers, the formation of nitroxyl radicals is accompanied by changes of macromolecular characteristics which permit us to explain the reinforcement as well as the modification of viscoelastic and physical-mechanical properties of crude and cured compounds. On the basis of assumptions made on the whole set of reactions regaxding both polymer modification with nitroso derivatives and that of rubber compounds, one can explain not only some of the properties but also the conditions in which modification should be made in order to obtain an optimal reinforcing effect.